1. Field of the Invention
The present invention relates generally to crawler-belt type vehicles, and more particularly to an improvement in "half-crawler vehicles" which include front wheels with pneumatic tires and rear crawlers.
2. Description of the Related Art
The so-called half-crawler vehicles are known from, for example, Japanese Patent Laid-open Publication No. 60-143189 disclosing an "off-road vehicle" and Japanese Patent Laid-open Publication No. 59-164270 disclosing a "four-wheel drive vehicle".
The off-road vehicle disclosed in the above-mentioned 60-143189 publication comprises a pair of left and right balloon-tired front wheels attached to the front portion of the vehicle's base frame structure or chassis, a pair of left and right rear balloon-tired wheels attached to the rear portion of the chassis, a pair of left and right idler wheels (intermediate wheels), and a pair of left and right rubber-made crawler belts each passing around and operatively connecting the associated rear wheel and idler wheel.
The four-wheel drive vehicle disclosed in the above-mentioned 59-164270 publication comprises a pair of left and right front wheels attached to the front portion of the chassis, a pair of left and right rear balloon-tired wheels attached to the rear portion of the chassis, a pair of left and right idler wheels, and a pair of left and right rubber-made crawler belts each passing around and operatively connecting the associated rear wheel and idler wheel. This four-wheel drive vehicle also includes a pair of left and right lower tired rollers (functioning as equalizers) each provided between the associated rear wheel and idler wheel. Each of the lower rollers is attached to the chassis via a support bracket and abuts against the inner surface of the associated crawler belt to press the belt against the road surface or ground.
Generally, with these half-crawler vehicles as disclosed in the 60-143189 and 59-164270 publications, high controllability is required on soft grounds such as a snow-covered road or ground, and high running performance is desired even on a fresh- snow-covered untrod ground.
In order to meet these requirements, it is necessary to appropriately examine and set the ground pressure with which the front wheel tires and crawler belts contact the ground. When the ground pressure of the front wheel tires is relatively low, these tires sink into a snow-covered or muddy ground with a small sinking depth, which would lead to each front wheel tire sinking into the soft ground over a relatively small area thereof as viewed sideways (projected area of sunk front-wheel portions). This, in turn, provides a small steering resistance, so that there can not be produced a necessary frictional force for properly steering of the front wheel tires. Too small steering resistance would make it difficult to provide sufficient controllability over the vehicle on a soft ground.
When the ground pressure of the front wheel tires is relatively high, these tires sink into a snow-covered or muddy ground with a great sinking depth and are subjected to increased resistance of the snow or mud, which would lower the sharp-turning capability of the vehicle. This is often a serious problem particularly on a fresh-snow-covered untrod ground.
Similarly, when the ground pressure of the crawler belts is relatively low, these belts sink into a snow-covered or muddy ground with a small sinking depth, while when the ground pressure of the crawler belts is high, these belts sink into a snow-covered or muddy ground with a great sinking depth. Too great sinking depth of the crawler belts would lead to a great running resistance and hence lower running performance of the vehicle. This is often a serious problem particularly on a fresh-snow-covered untrod ground.
Further, in the off-road vehicle disclosed in the 60-143189 publication, intermediate wheels functioning as driving wheels are disposed between the front wheels and the idler wheels, and a vehicle engine is provided between the front wheels and the intermediate wheels.
Generally, the half-crawler vehicles, running on snow-covered or muddy soft grounds, require a large driving force of their crawlers and hence are equipped with a large-sized and heavyweight engine. Thus, in each of such half-crawler vehicles, the mounted position of the heavyweight engine would have a great effect on the wheel load (part of the vehicle's weight acting on the wheels).
Furthermore, because the vehicle's running performance on the snow-covered ground depends on the ground pressure of the front wheels and crawler belts as noted earlier, the wheel load would have a significant effect as a factor determining the ground pressure. Namely, it is absolutely necessary to minimize the overall weight of the vehicle, in order to allow the vehicle to appropriately run on a snow-covered, muddy or other sort of soft ground.
As another example of the half-crawler vehicle, a "tire-driven crawler belt" is disclosed in Japanese Utility Model Publication No. HEI-8-8891. On the inner surface of each of the vehicle's crawler belts, there is formed a pattern of raised and recessed portions for contact with the treads of the tires. The pattern of raised and recessed portions forms a multiplicity of oblique channels as escapeways for muddy water. Namely, water and mud drawn between the tire treads and the inner surface of the crawler belts are forced out or discharged along the oblique channels by the channels narrowing in width as the crawler belts bend along the curved surface of the tires and also by the pattern of raised and recessed portions being compressed by the tires. The disclosed half-crawler vehicle permits an efficient escape of muddy water and such, in a situation where the crawler belts do not sink deep into the soft ground.
However, the crawler belts, running on a snow-covered, muddy or other sort of soft ground, often sink deep into the soft ground. In such a case, snow or mud drawn between the tire treads and the crawler belts can not be let out efficiently. Reduced efficiency in letting out the snow or mud would reduce frictional resistance between the tires and crawler belts, which in turn would result in reduced efficiency in driving force transmission from the tires to the crawler belts.
In particular, on a fresh-snow-covered untrod ground, the tires and crawler belts of the half-crawler vehicle sink deep into the soft ground, so that a great amount of snow may be drawn into the inner surface area of the crawler belts. If the snow is drawn into and compressed between the tires and the crawler belts, it may easily turn into ice, which would significantly reduce the frictional resistance between the tires and the crawler belts. Further, the tires and crawler belts, sunk deep into the fresh-snow-covered untrod ground, would present increased running resistance, so that the vehicle requires a greater driving force. The reduced frictional resistance between the tires and the crawler belts would have a significant adverse effect on the running performance of the half-crawler vehicle.
Furthermore, in the above-mentioned crawler belt vehicle disclosed in Japanese Patent Laid-open Publication No. 59-164270, each of the lower rollers presses the associated crawler belt in a point-to-point contact, and hence the ground contact area of the crawler belt is quite small. In order to increase the ground contact area of each of the crawler belts to thereby attain an increased ground adhesion force ("grip"), it is necessary to provide a relatively large number of the lower rollers in the longitudinal direction of the crawler belt, which would however increase the structural complexity of the vehicle.
As one possible approach to increase the ground adhesion force with a relatively simple construction, there has been proposed a crawler as shown in FIG. 25. In this proposed crawler 100, a rubber-made crawler belt 103 passes around and operatively connects two vehicle wheels 101 and 102, and the crawler belt 103 is pressed against a ground via a slider 104 (equivalent to the equalizer) of predetermined length normally urged downward by a spring 105. By the crawler belt 103 being pressed against the ground via the slider 104 of predetermined length, the ground contact area and adhesion force can be increased to a considerable degree.
However, the proposed crawler 100 presents the problem that frictional resistance between the crawler belt 103 and slider 104 undesirably increases because they are constantly in sliding contact with each other. Increase in the frictional resistance between the crawler belt 103 and slider 104 leads to increased resistance to the movement of the crawler belt 103, which would shorten the life of the crawler belt 103 due to the resulting frictional heat. The frictional resistance between the crawler belt 103 and slider 104 can be lowered by only reducing the urging or pressing force of the spring 104, but such reduced pressing force of the spring 104 can not achieve a sufficient ground contact force of the crawler belt 103.
An improved crawler belt addressing the foregoing problems is disclosed in, for example, Japanese Utility Model Publication No. 52-28258. This improved crawler belt has a plurality of side guide protrusions formed thereon along the opposite longitudinal edge portions and spaced apart from each other in the longitudinal direction of the belt. U-shaped reinforcing metal pieces are embedded in the individual side guide protrusions as well as in flat belt portions between the protrusions. The side guide protrusions function to prevent the tires from being accidentally detached from the crawler belt, and the reinforcing metal pieces function to increase the rigidity of the crawler belt against its lateral bending.
When the side guide protrusions are subjected to a force, applied from the tires, pressing the crawler belt in the laterally outward direction, a bending moment occurs at the base of the guide protrusions. Bending rigidity of the reinforcing metal pieces has a direct effect on that of the side guide protrusions. Therefore, in order to effectively avoid accidental detachment of the tires from the crawler belt, it is necessary to enhance the bending rigidity of the reinforcing metal pieces. The bending rigidity may be enhanced by increasing the thickness of the metal pieces, which would however result in an increased thickness (base gauge) of the crawler belt. The increased thickness would make the crawler belt difficult to bend, thus increasing the running resistance to the crawler vehicle.
Further, in the half crawler vehicle disclosed in Japanese Patent Laid-open Publication No. 59-164270, the chassis and wheels are connected with each other via a suspension including springs or dampers. When the half crawler vehicle is travelling along a slope, the chassis inclines laterally so that a portion of the body above the suspension is displaced toward the bottom of the slope. As a consequence, the vehicle would lose the weight balance and can not be operated appropriately.
In addition, the crawler belts of the half-crawler vehicles generally suffer from considerable resistance to a sideslip, and this sideslip resistance would act as a braking factor and reduce the vehicle's small-turning capability.
Further, according to the disclosure of the above-mentioned HEI-8-8891 publication, the half-crawler vehicle includes front wheels with pneumatic tires mounted on a front portion of the chassis, rear wheels with pneumatic tires mounted on a rear portion of the chassis, intermediate wheels with pneumatic tires disposed between the front and rear wheels, and half crawler belts each passing around and operatively connecting the associated front and rear wheels. This half-crawler vehicle is based on a front-and-rear-wheel (four-wheel) drive where the driving force from the vehicle engine is delivered to both the front wheels and the rear wheels.
The disclosed half-crawler vehicle is designed in such a manner that the crawler belts are attached to the intermediate and rear wheels only when the vehicle is to run on an off-road such as a soft ground; for normal smooth roads, the crawler belts can be removed or detached from the wheels so as to afford better running performance and riding comfort. However, the effective radial length of each of the rear and intermediate wheels, measured from the wheel's center to the ground, significantly varies depending on whether the crawler belt is attached thereto or not; namely, the effective radial length varies exactly by the thickness (base gauge) of the crawler belt--a greater thickness of the crawler belt yields a greater variation in the effective radial length.
Additionally, because the half-crawler vehicle disclosed in the HEI-8-8891 publication is based on the front-and-rear-wheel drive, it is necessary that the rotating speeds of the front and rear wheels be adjusted to accurately coincide with each other when the crawler belts are removed. In other words, if the rotating speeds of the front and rear wheels do not agree with each other, some effective measures must be made between the driving systems for the front and rear wheels to avoid inconveniences.
Also, the front wheel tires may be replaced with larger-diameter tires in order to enhance the running performance of the vehicle in accordance with the running surface conditions. In such a case as well, some effective measures must be made between the driving systems for the front and rear wheels.